High molecular weight vinyl aromatic polymers, particularly polymers having weight average molecular weights (Mw) of greater than 300,000, have been typically produced by anionic polymerization rather than by free radical polymerization due to the slow polymerization rates used in free radical techniques to achieve high molecular weight polymers. However, anionic polymerization processes require expensive anionic initiators, such as organolithium compounds, and tend to produce discolored products due to the presence of residual lithium-containing salts.
High molecular weight vinyl aromatic polymers have also been produced by free radical polymerization in the presence of a soluble organic acid having pKa of 0.5 to 2.5, as in U.S. Pat. No. 5,145,924. However, in this process the acid does not bind to the polymer can migrate from the polymer during use, which can cause corrosion of mold surfaces.
It is also known to produce bimodal compositions containing high molecular weight monovinylidene aromatic polymers having Mw of about 400,000 to about 1,900,000 and intermediate molecular weight monovinylidene aromatic polymers having Mw of about 100,000 to about 240,000. These compositions have good combinations of strength, heat resistance and melt flow properties as described in U.S. Pat. No. 4,585,825. However, the high molecular weight polymer of this composition is produced at low temperatures and low conversion rates which require very long reaction times, such as 26 hours at 90.degree. C. to produce 800,000 Mw polystyrene.
Accordingly, it remains highly desirable to produce high molecular weight polymers from vinyl aromatic monomers using a free radical polymerization process which does not exhibit the foregoing disadvantages, as well as to provide an improved process to produce bimodal compositions containing these high molecular weight polymers.